To investigate the effect on the immune response of Sendai viral infection, several classical in vitro immunologic parameters were studied. Detailed time courses and dose responses of Sendai viral infection reveal no suppression of the immune response as measured by responses of spleen cells from infected mice to mitogens, proliferative mixed lymphocyte reaction, and cell-mediated lympholysis. Studies done in syngeneic normal mouse serum or sera from Sendai infected animals showed a moderate enhancement of the in vitro immune response to mitogens and alloantigens by serum from infected animals.